1. Technical Field
This invention relates to semiconductor devices, and more particularly to a method for testing programmable memory devices for data retention characteristics.
2. Description of the Related Art
Electrically-erasable, programmable read-only memory devices (EEPROMs) operate on the basis of storing charge on a floating gate. The gate is floating in that it is not connected to any other electrode or to the silicon substrate, but instead is isolated from any conducting path by silicon oxide. The ability of the floating gate to store charge without leakage determines the ability of the EEPROM device to store data over a long period of time. Preferably, an EEPROM device can store data for many years.
In a manufacturing process for devices of this type, many factors can lead to reduction of the quality of the silicon oxide used to isolate the floating gates, and thus lead to deterioration of the data storage ability over time. The devices must be tested at the end of the manufacturing process to insure that such deterioration has not occurred or is at acceptable levels so data retention is within the specifications for the devices. But since the specified data retention periods are measured in years, it is not possible to life-test the devices or even samples of the devices before delivery to customers.
The screening method ordinarily used for EEPROM devices employs a high-temperature bake. High temperature accelerates the loss of charge stored on a floating gate. Devices are subjected to bakes for periods from a few hours to a few days. The amount of charge lost from the floating gates is of course dependent on time and temperature, but the temperature cannot be higher than certain limits or the devices are destroyed. Increasing the time to a point where a reliable test is obtained is expensive in that costly and bulky equipment is required which must be tied up for long periods, and this test interrupts the flow of material in the manufacturing process. Only a sample may be tested in this way, or preferably 100% of the devices are screened using this heat treatment. And, this screening is done after the devices are packaged rather than when still in wafer form, further increasing cost.
Certain types of programmable memory devices are erased by ultraviolet light (UV). In addition to being erasable electrically, an EEPROM device could be erased by UV, and indeed EEPROMs (i.e., floating gate devices which were not electrically erasable) were erased only by UV, and had a window in the package above the chip to allow UV to impinge upon the chip. Indeed, the effect of UV on loss of charge is greater than that of heat, so a device can be totally erased in a few seconds using UV, whereas the same device might take orders of magnitude longer to become totally erased by heat.
Heretofore, testing or screening equipment for EEPROMs using UV instead of heat for forcing loss of charge has not been available.